Vapor-liquid-solid nucleation of GaAs on Si(111): Growth evolution from traces to nanowires

S. Breuer, M. Hilse, A. Trampert, L. Geelhaar, and H. Riechert

Phys. Rev. B 82, 075406 – Published 6 August 2010

Abstract

GaAs nanowires were grown on Si(111) substrates by molecular-beam epitaxy employing Au droplets for the vapor-liquid-solid mechanism. The nucleation happens in three stages, which coincide with the abundance of one of three different GaAs manifestations: first Au-induced lateral traces, then three-dimensional islands, and finally Au-induced vertical nanowires. During deposition of the first 7 ML of GaAs no nanowires grew. By reflection high-energy electron diffraction and transmission electron microscopy, the crystal structure of the GaAs manifestations was examined. Traces and islands adopted the zinc-blende crystal structure and include twins, while nanowires grew in the hexagonal wurtzite structure. The delay in nanowire formation was observed to increase for higher growth temperatures. The gradual covering of the Si by GaAs was comparable to the case of Au-free growth and appears to be linked with the evolution from trace to nanowire growth. During trace formation, Au droplets kept in contact with the Si substrate and were pushed sideways by precipitating GaAs. The nucleation stages could thus be explained by considering that any Au droplet created a trace while in contact with the Si surface and a nanowire after having been pushed onto GaAs. This mechanism of trace growth was explained by the lower interface energy of liquid Au droplets on Si when compared with that on GaAs, as supported by numerical estimates.

Received 25 February 2010

DOI:https://doi.org/10.1103/PhysRevB.82.075406

Authors & Affiliations

S. Breuer^*, M. Hilse, A. Trampert, L. Geelhaar, and H. Riechert

Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany

^*steffen.breuer@pdi-berlin.de

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Issue

Vol. 82, Iss. 7 — 15 August 2010

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Images

Figure 1
(Color online) (a) Four RHEED patterns showing: the $(1 \times 1)$ As-terminated Si(111) surface before growth, the pattern of ZB GaAs with twins after 5 s, the superposition of ZB and WZ GaAs after 100 s, and the WZ GaAs diffraction pattern after 500 s which remains unchanged until growth ends. (b) The time-dependent intensities of a ZB and a WZ spot give evidence for a gradual change from nucleation in the ZB to growth in the WZ crystal structure. The resulting three growth stages are indicated. The two analyzed spots are marked in (a): ZB ${(1 \bar{1} \bar{1})}_{T}$ by squares and WZ $(01 \bar{1} 0)$ by circles.Reuse & Permissions
Figure 2
(Color online) (a) SEM images in $45 °$ incidence of samples grown at $500 ° C$ for different durations. All scale bars indicate 100 nm. The following GaAs structures were identified: traces after 5 s, traces and islands after 60 s, islands and nanowires after 300 s, and nanowires on a coalesced layer after 1800 s. (b) The plot shows the percentage of the Si surface area covered by GaAs and the number density of nanowires versus growth time. The three stages are indicated.Reuse & Permissions
Figure 3
(Color online) TEM micrographs at different regions of the same sample grown for 150 s at $500 ° C$ : (a) GaAs trace with frontal Au droplet; (b) same trace in high resolution showing the ZB crystal structure with twins and the crystallized Au droplet; (c) 3D GaAs island, structurally ZB with twins; and (d) WZ GaAs nanowire with Au droplet at the tip and a ZB GaAs base layer. All scale bars indicate 5 nm. The insets show 2D Fourier transformations of selected regions for their crystal structure identification.Reuse & Permissions
Figure 4
(Color online) (a) SEM images in $45 °$ incidence of samples grown for 150 s at various substrate temperatures: at $350 ° C$ the Si substrate was covered by GaAs and many nanowires grew; at $450 ° C$ GaAs had formed islands, traces on remaining Si and nanowires on the GaAs islands; at $550 ° C$ island growth was confined to a smaller total area, many traces grew, and nanowires did not form; and at $600 ° C$ only islands grew. (b) Plot of the temperature dependence of the area covered by GaAs and the nanowire density. Both show an essentially parallel trend with temperature.Reuse & Permissions
Figure 5
(Color online) Schematic of (a) trace growth, (b) transition from trace to nanowire growth, and (c) nanowire growth. While the GaAs growth front is at the droplet-GaAs interface, the droplet-Si interface has a lower interface energy. The resulting growth directions are indicated by arrows.Reuse & Permissions

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